Cells-GTK is a binding to GTK+ that provides an object/components framework for GUI construction (similar to CAPI) and a declarative "rule-based" means to keep the state of the GUI consistent with the state of the running application.

Cells-gtk is a complete, portable framework for Common Lisp GUI development via GTK+2 with Cells InsideTM, licensed under the Lisp LGPL. It transparently link GUI elements with each other and the application model to greatly simplify development of rich interfaces. Cells automates how Lisp GUI instances drive their GTK+ counterparts.
Cells-GTK contains a very thorough example program that exercises nearly every GTK widget implemented.